In the art of extractive metallurgy, "cement copper" is the term used to identify the product obtained by precipitating copper from solutions, generally with added iron as precipitant. There is no known method to continuously firesmelt and refine cement copper to produce metallic copper with a purity similar to that resulting from electrolytic methods, e.g. 99.9%. Conventional methods used industrially are very complex; a reverberatory furnace is charged with sulfide concentrates of low purity, 40% to 48%, the smelting of which produces a matte consisting of double copper and iron sulfide, with a 45% to 55% Cu content. This matte is loaded into a converter, wherein it is oxidized with air or oxygen to obtain 98.5% Cu blister copper. This blister copper is then fire refined, cast into anodes, and refined electrolytically to produce 99.9% Cu cathodes.
No commercially known process is known for the continuous smelting and refining of cement copper, in such a way to permit full use of its original high purity.
The present invention is directed to a method for obtaining metallic copper with a purity of 99.9% by direct fire smelting and refining of cement copper in a specially designed shaft kiln in a single, continuous operation. The present invention is advantageously employed in conjunction with the continuous, high-purity process for producing granular cement copper described in U.S. Pat. No. 3,874,940, assigned to the same assignee as the instant application.
The shaft kiln is a vertical furnace traditionally used for smelting iron, scrap iron or pig iron, and which is provided with nozzles or tuyeres at its lower end. It uses metallurgical coke as fuel and its interior carries a lining of refractory material. Such a kiln has three main parts:
(A) The lower section or hearth of the kiln, where the smelt metal descending from the charge in the shaft is collected. This part is provided with a lower outlet or tap to allow outflow of the melt.
(B) The intermediate section, situated immediately above the hearth is the area of the kiln exhibiting the highest temperature, and at its lower end are the tuyeres and wind boxes, through which air is blown in.
(C) The highest section of the kiln, above the midsection, is where the loading gates or chutes are situated to receive the ore, coke, and flux.
In order to smelt metal, the shaft kiln uses the heat irradiated by an incandescent coke column that is permeable to gases. It is provided with combustion ignition and maintaining systems, and air is blown in through the tuyeres. Coke and iron scrap or whatever are loaded through the charging gates in alternate layers that descend progressively to the intermediate section to the extent that the fuel is consumed, and the metal completes smelting at this area of higher temperature. The smelt metal drips through the incandescent coke and deposits itself on the hearth.
It is impossible to smelt cement copper in a conventional shaft kiln, because on loading the cement through the upper gates, it is drawn by the gas current originating in the combustion zone and blown out of the furnace. Nor is it possible to load intermittently significant amounts of cement, because its fine granulation obstructs the permeability of the incandescent column, causing the kiln to extinguish. Although in theory briquetted cement copper could be loaded alternatively with the coke, in practice this is not practical because the added cost of manufacturing briquettes raises costs to noncommercial levels.
The present invention provides for loading the cement mixed with flux directly to the intermediate section of the shaft kiln, avoiding the indicated inconveniences of loading through the upper gates, and without obstructing the normal operation of the kiln. To this effect, specific improvements on the conventional shaft kiln have been designed, enabling cement copper smelting and refining to be carried out in a continuous process.